This invention relates to refractory fiber compositions useful in forming fibrous thermal insulation for the reduction of heat loss behind brick lined furnaces, or as a primary lining in reheat furnaces where relatively low gas velocities are encountered or contact with molten slag does not occur.
Ceramic fibers of silica (SiO.sub.2) alumina (Al.sub.2 O.sub.3) and zirconia (ZrO.sub.2) having sufficient refractoriness to retain their strength and resilience at temperatures of 2000.degree.-2400.degree. F. are known and have been used commercially to provide thermal insulating materials. These refractory fiber materials can be produced by the blowing method, i.e. impinging a high velocity stream of gas against a falling stream of molten inorganic composition, but are more typically produced by the spinning method. In the latter method a molten stream of inorganic material is released onto either a side or the periphery of one or more rapidly rotating cylinders which throw the molten material off in a tangential direction thereby fiberizing it.
Regardless of the method used to form the ceramic fibers the fibers as formed are in the glass state, i.e. they are amorphous or non-crystalline. The formation of thermal blankets and other products from these fibers does not affect the state of the fiber. As the prior art fibers are heated over time, for example when used in the form of a blanket for a furnace lining, the silicate mineral mullite (3Al.sub.2 O.sub.3 .multidot.2SiO.sub.2) is formed at temperatures above 1700.degree. F. At temperatures above about 2200.degree. F. the mineral cristobalite (SiO.sub.2) forms. Recently, a concern has arisen that cristobalite is a human carcinogen and therefore efforts have been made to eliminate it from the workplace.